In recent years, with an increase in the recording density of optical discs, the length of the shortest recording mark is nearing the limit of optical resolution, and an increase in intersymbol interference and deterioration of signal-noise ratio (SNR) have become more noticeable. Therefore, a partial response maximum likelihood (PRML) scheme is generally used as a signal processing method.
The PRML scheme is a technique that combines partial response (PR) and maximum likelihood (ML) decoding and is a method of selecting a most probable signal sequence from a reproduced waveform on the assumption that known intersymbol interference occurs. Thus, it is known that decoding performance is improved as compared to a conventional level determination method.
Signals reproduced from an optical disc are subjected to partial-response equalization using a waveform equalizer, a digital filter, and the like so as to have predetermined frequency characteristics and are then decoded into corresponding binary data by selecting a most probable state transition sequence using the Viterbi decoding or the like. A value “L” that indicates a probability where a state transitions to a state “Sn” (n is a state number) until time “k” is defined by Expression (1) below.
                    L        =                              ∑                          i              =              0                        k                    ⁢                                    (                                                y                  i                                -                                  E                  i                                            )                        2                                              (        1        )            
In Expression (1), “yi” is the value of a reproduced signal at time “i”, and “Ei” is the value of an expected ideal reproduced signal. In the maximum likelihood decoding scheme, such a state transition sequence that the value “L” indicating the probability obtained by Expression (1) is minimized is selected and is decoded into corresponding to binary data (for example, see Patent Literature 1).
As the increase in the density of optical discs progresses further, the deterioration of intersymbol interference and SNR causes severe problems. Reproduction performance can be maintained by using higher-order PRML schemes. For example, when the storage capacity per recording layer of a 12-cm optical disc is 25 GB, good reproduction performance can be maintained by employing a PR(1,2,2,1) ML scheme. On the other hand, when the storage capacity per layer is 33.3 GB, it is necessary to employ a PR(1,2,2,2,1) ML scheme. Actually, BDXL of which the storage capacity per layer is 33.4 GB is already in practical use, and a PR(1,2,2,2,1) ML scheme is employed (for example, see Non Patent Literature 1).
Moreover, high-speed optical communication techniques have been developed in order to deal with an abrupt increase in information traffic. With the increase in the information traffic, the input optical power has been increasing remarkably and optical communication techniques have reached the physical limit of optical fibers where nonlinear optical effect, thermal destruction, and the like occur. In order to improve optical communication efficiency, optical wavelength multiplex modulation schemes which use light having multiple wavelengths and techniques for improving the performance of dispersion compensation fibers that compensate for distortion resulting from dispersion of an optical signal waveform occurring due to propagation characteristics of optical fibers or the performance of optical fibers such as optical amplifiers, which mainly become transmitters and transmission paths are gathering attention.
The PRML scheme used in the optical discs assumes that reproduced signals can be equalized so as to have a predetermined frequency characteristic.
FIG. 12 is a diagram showing an optical transfer function (OTF) of BDXL and the frequency characteristics of PR(1,2,2,2,1). The OTF gain becomes zero when a normalized frequency is 0.23. In contrast, the gain of the PR(1,2,2,2,1) characteristics becomes zero temporarily when the normalized frequency is 0.25 and becomes higher than zero in a frequency range where the normalized frequencies are higher than 0.25. The reproduced signal is equalized by a waveform equalizer so as to have the PR(1,2,2,2,1) characteristics. In this case, since the OTF gain is larger than zero in a frequency range where the normalized frequencies are smaller than 0.23, it is possible to equalize the reproduced signal. However, since the OTF gain is zero in a frequency range where the normalized frequencies are 0.23 or higher, it is not possible to amplify the amplitude characteristics of the reproduced signal.
FIG. 13 is a diagram showing the waveform of an ideal signal having the PR(1,2,2,2,1) characteristics and the waveform of a reproduced signal of BDXL equalized with a waveform equalizer. As shown in FIG. 13, since it is not possible to equalize signal components in a frequency range where the normalized frequencies are higher than 0.23, an error always remains in the amplitude values of the ideal signal and the equalized reproduced signal. Thus, even when a noise component applied to the reproduced signal is zero, the value “L” indicating the probability of state transitions shown in Expression (1) does not become zero, and the error causes deterioration of the decoding performance of the PRML scheme. Such deterioration of the decoding performance is called distortion deterioration. In the PR(1,2,2,2,1) characteristics shown in FIG. 12, signal components of which the normalized frequencies are higher than 0.23 and which result in distortion deterioration take approximately 11% of all signal components of the PR(1,2,2,2,1) characteristics.
FIG. 14 is a diagram showing an optical transfer function of an optical disc of which the linear recording density is twice that of BDXL and the frequency characteristics of the PR(1,2,3,3,3,3,3,3,3,2,1) characteristics suited for this optical disc. The OTF gain becomes zero when the normalized frequency is 0.11. The gain of the PR(1,2,3,3,3,3,3,3,3,2,1) characteristics becomes zero temporarily when the normalized frequency is 0.11 similarly to the OTF gain and becomes higher than zero in a frequency range where the normalized frequencies are higher than 0.11. In this case, signal components which result in distortion deterioration take approximately 19% of all signal components of the PR(1,2,3,3,3,3,3,3,3,2,1) characteristics, and the influence of distortion deterioration increases nearly twice as compared to when the PR(1,2,2,2,1) characteristics were applied to the linear recording density of the BDXL.
FIG. 15 is a diagram showing an optical transfer function of an optical disc of which the linear recording density is twice that of BDXL and the frequency characteristics of PR(1,2,3,4,5,6,6,6,5,4,3,2,1) characteristics in which the gain in a high frequency range where the normalized frequencies are 0.11 or higher is decreased. Due to the decreased gain in the high frequency range, although signal components which result in distortion deterioration can be reduced up to approximately 7% of all signal components, it is not possible to make distortion deterioration zero.
In order to make distortion deterioration zero, a target PR class needs to have exactly the same frequency characteristics as those of OTF. However, since the OTF always changes depending on the reproduction conditions, if the PR class is determined to have a single frequency characteristic, distortion deterioration occurs. That is, it can be said that equalization of a reproduced signal to a frequency characteristic of a predetermined PR class becomes the cause of distortion deterioration.
Moreover, a waveform equalizer amplifies a signal amplitude so that the frequency characteristic of a reproduced signal approaches the frequency characteristic of a set PR class. In this case, the amplification factor in a high frequency range is likely to increase in order to equalize the errors in a frequency range higher than the normalized frequency at which the OTF gain becomes zero. As a result, the noise component in the high frequency range of the reproduced signal is amplified too much, and the decoding performance of the PRML scheme is deteriorated.
As described above, in the conventional PRML scheme, there is a problem in that as with the increase in the density of optical discs, the decoding performance deteriorates due to the influence of distortion deterioration and amplification of noise components in the high frequency range, and it is not possible to reproduce data recorded on an optical disc.
Moreover, the efficiency of optical communication has been improved using the above-described optical wavelength multiplex modulation scheme, dispersion compensation fibers, optical amplifiers, and the like. Further, in addition to this, a communication speed can be increased by increasing an optical modulation speed to use a broader frequency range in order to increase the amount of information transmitted per wavelength of light. However, in order to accurately receive the waveform of an optical signal modulated using a broader frequency range, it is necessary to improve an optical detector used in a receiver. An optical detector converts a received optical signal waveform into an electrical signal. In an optical detector, the frequency range where optical signals can be efficiently converted is limited. Thus, there is a problem in that even when an optical modulator on the transmitter side modulates signals using a broad frequency range, the optical detector cannot detect the signals sufficiently, and the use of the broad frequency range becomes an obstacle to improvement of the communication speed. Moreover, although this problem can be solved by increasing the sensitivity of the optical detector, there is a problem in that the size of the optical detector increases and the power consumption also increases.